1. Field of the Invention
The present invention relates to a method of obtaining a parameter from nuclear magnetic resonance measurement data. Specifically, the invention improves log acquisition efficiency and formation evaluation in stacked signals obtained from s multiecho sequences from various regions of magnetic field gradient.
2. Description of the Related Art
A new generation of multi-frequency nuclear magnetic resonance (NMR) logging instruments is capable of acquiring data useful for characterizing both formation rock properties (e.g., porosity, bound and movable fluids, and permeability) and reservoir fluid properties. However, acquiring data for both of these characterization goals often requires a diverse assortment of NMR acquisition parameters and sequences. High-resolution formation characterization requires acquisition schemes that generate high S/N echo data without compromising vertical resolution. Fluid properties usually vary more slowly with depth than rock properties, but obtaining fluid properties requires NMR echo data acquisition that maximizes fluid contrasts (e.g. differences between gas, oil, and water). Said data acquisition is best achieved by optimally varying combinations of the magnetic field gradients (G), the inter-echo spacing (TE), and the polarization time (TW) in the acquisition scheme.
NMR logging is based on the static and dynamic aspects of nuclear spins in the presence of a static magnetic field and under the influence of RF excitations. When an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field, they tend to align along the direction of the magnetic field, resulting in a bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter known as the spin-lattice relaxation time, T1. Another related and frequently used NMR logging parameter is the spin-spin relaxation time T2 (also known as transverse relaxation time), which is a characteristic decay time due to inhomogeneities in the local magnetic field over the sensing volume of the logging tool. Both relaxation times, along with the magnetization strength, provide information about the formation porosity, the composition and quantity of the formation fluid, and other parameters. Methods of obtaining NMR measurements in a downhole environment are described in prior art. Useful techniques and apparati for carrying out such techniques are described in U.S. Pat. No. 4,717,877, issued to Taicher and U.S. Pat. No. 4,710,713, issued to Strikman.
Another measurement parameter obtained in NMR logging is the diffusion of fluids in the formation. Generally, diffusion refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. Self-diffusion is an important parameter, since it is inversely related to the viscosity of the fluid, which is a parameter of considerable importance in borehole surveys. In a uniform magnetic field, diffusion has little effect on the decay rate of measured NMR echoes. In the presence of a gradient magnetic field, although diffusional motion is the same as in the case of no field gradient, the rate of dephasing of a nucleus is significantly greater, thereby resulting in a faster rate of decay. This decay rate is dependent on G2 D, where G is the magnetic field gradient and D is the value of diffusivity.
One important petrophysical parameter that can be derived from NMR logs is the bound volume irreducible, BVI. A commonly used method for estimating BVI is described in the prior art. The method uses a T2 cutoff value for computation such that                               B          ⁢                                           ⁢          V          ⁢                                           ⁢          I                =                              ∫                          T                              2                ⁢                                                                   ⁢                min                                                    T                              2                ⁢                cutoff                                              ⁢                                    P              ⁡                              (                                  T                  2                                )                                      ⁢                          ⅆ                              T                2                                                                        (        1        )            where P(T2) is the apparent T2 distribution with individual T2 component expressed by                               T          2                      -            1                          =                              T                          2              ⁢                                                           ⁢              B                                      -              1                                +                      ρ            ⁢                          S              V                                +                                    T                              2                ⁢                                                                   ⁢                Diff                                            -                1                                      .                                              (        2        )            where T2B is the relaxation of the bulk fluid, ρ is the surface relaxivity, S is the pore surface area, V is the pore volume, and T2 Diff is additional decay time due to diffusion effects. The diffusion term                               T                      2            ⁢                                                   ⁢            Diff                                -            1                          =                                            γ              2                        ⁢                          G              2                        ⁢            T            ⁢                                                   ⁢                          E              2                        ⁢                          D              fluid                                12                                    (        3        )            depends on (a) field gradient G, which associates with the acquisition frequency; (b) inter-echo time TE, an acquisition parameter; (c) diffusivity D, a fluid property, and, (d) the gyromagnetic ratio, a property dependent on the nuclear species. In NMR logging, usually only proton spin is of interest. Of note is the multiplicative factor G*TE, consisting of two parameters alterable by the operator. Obviously, data that is acquired using a different G*TE factor will result in a different apparent T2 distribution, P(T2). Equation (1) indicates that if a same T2 cutoff value is used to compute BVI of the same formation, the results P(T2) may be dependent on gradient and TE. If the value of G*TE is small, then the T2Diff−1 term contributes significantly less to T2−1 in Eq. (2) than either the bulk fluid relaxation term T2B−1 or the surface relaxation term ρ(S/V). In such a case, the dependency of T2−1 on diffusion may be negligible. However, if the value of G*TE is large, T2Diff−1 can become the dominant contributing term to T2−1.
Without considering the tool and acquisition dependencies, the value of T2 cutoff depends on the rock surface mineralogy. A common practice is to “calibrate” the T2 cutoff from laboratory-based NMR measurements. These lab measurements are often carried out in a magnetic field setup in which no external gradient is applied and thus for which there is a negligible contribution of T2 Diff−1. This is in disparity with the external gradient found in logging tool measurements. The combined echo train enables the use of a single T2 cutoff consistent with laboratory core-NMR derive T2 cutoff, rendering a better means for core-log integration.
U.S. Pat. No. 5,212,447, issued to Paltiel describes a method and apparatus for determining the self-diffusion constant of earth formations penetrated by a wellbore. Paltiel '447 discloses a technique for conducting borehole NMR measurements including the steps of providing a magnetic field gradient at a desired location along a borehole, obtaining at least one and preferably two or more sets of NMR data in the presence of the magnetic field gradient, sensing the diffusion effect on the decay of at least the first echo and determining therefrom the diffusion coefficient. Obtaining at least one set of NMR data includes carrying out two sets of NMR data acquisitions such that the sets differ in at least one of the following parameters: the time the molecules are allowed to diffuse, the magnitude of the magnetic field gradient, and the time over which the pulses are applied (if magnetic field gradient pulses are used).
U.S. Pat. No. 5,698,979, issued to Taicher discloses a method of measuring motion properties of nuclei within pore spaced of a porous medium. The method includes applying a static magnetic field to the medium to polarize the nuclei, generating a first magnitude of magnetic field gradient within the pore spaces of the medium, applying a radio frequency magnetic field to excite the nuclei receiving NMR signals from the nuclei, and calculating the motion properties from rates of decay of the amplitude of the NMR signals. Taicher '979 applies a static magnetic field having a first amplitude, a second amplitude and an amplitude gradient, and sequentially excites nuclei and receives resonance signals at frequencies corresponding to regions defined by the first and second magnetic amplitudes. Motion calculation is determined from differences in rates of decay of the amplitudes of the resonance signals from the first and second frequencies.
U.S. Pat. No. 6,316,940, issued to Akkurt, discloses a method of separating signals from different fluids using user-adjusted measurement parameters. Akkurt '940 is based on forcing diffusion as the dominant relaxation mechanism for the brine phase in NMR measurements of a geologic formation. Certain measurement parameters are changed to enhance the role of diffusion relaxation in the brine phase. The enhanced diffusion relaxation in turn establishes an upper limit for the T2 distribution of the brine phase, which limit can be calculated. Once this upper limit is found, any phase having a T2 longer than the upper limit can be identified unambiguously as not being brine. The measurement parameters that are varied are the inter-echo time TE and the magnetic field gradient G of the tool.
U.S. Pat. No. 6,377,042 issued to Menger, discloses a method and system to obtain enhanced-resolution NMR data by merging, in the time domain, different NMR pulse echo trains into a single echo train. The input echo trains can be acquired with different inter-echo spacing, wait time, and signal-to-noise ratio parameters that are optimized to correspond to both fast and slow portions of the T2 spectrum. The merged echo trains are inverted into complete T2 spectra in a single step thereby overcoming ambiguities and other limitations of prior art methods. In a preferred embodiment, the merging process does not require a priori information about T1, and the merged echo trains are optimized in with respect to T2 resolution. The method of Menger '042 discloses inverting and binning input data including partially recovered and fully recovered data. In a second step, the difference between the invented data is calculated for all bins within a certain range, enabling calculation of an “artificial” echo train, which can be added to the original partially recovered data. In a third step, data is merged to obtain a final echo train, which is provided as an input for standard T2 inversion. In order to obtain a more complete knowledge of rock formation, it is necessary to consider as many parameters as possible, including changes concerning the static magnetic field (i.e. field gradient). Menger '042 address changing echo train parameters, but does not address the effect of a change in the field gradient parameter.
To date, multiple G, TE and TW data are not combined in time-domain processing to obtain formation rock properties. The main obstacle is that different G-TE data cannot be simply stacked. A common practice, dictated in part by hardware limitations of older NMR tools, has been to log multiple passes, each with separate evaluation objectives. Even with multifrequency tools that are capable of acquiring comprehensive data in a single pass, data are not used economically. Thus, in order to satisfy the formation rock and fluid property characterization requirement, one is forced to either log slowly or to compromise the vertical resolution.
Clearly there is a need to develop a method that improves log acquisition efficiency and maximizes economic usage of all data. The present invention addresses the above-mentioned problem.